Quite easy. Produce a PDF with non-standard fonts, but don’t embed them. Whoever opens it without those fonts installed will see glyphs from an installed default font, positioned where the glyphs of the original font would have been.

This has happened to me when using older versions of LibreOffice on RHEL.
Maybe it was the bad font in RHEL, or maybe it was some old bug that got fixed later.

Electroboom recently published a video on the snakeoil of grounding products like these.